1. Field of the Invention
The present invention relates to a semiconductor optical element and an integrated semiconductor optical element applied to, for example, an optical communications system.
2. Description of the Related Art
In recent years, with regard to a semiconductor optical element, application thereof in which, from the viewpoint of power savings, temperature rise suppression of the element, and the like, high efficiency use of injected current is required, and, at the same time, operation at higher speed than ever is required is increasing. For example, when the semiconductor optical element is a buried semiconductor laser, for the purpose of materializing high efficiency, in order to inject with efficiency current into an active layer for converting electricity to light, it is necessary to suppress a reactive current flow through a buried layer which is formed on both sides of an active layer and which has a current blocking function. Further, similarly, with regard to a buried semiconductor laser, in order to materialize high speed response characteristics, it is necessary to decrease a parasitic capacitance of the buried layer.
It is known that, in order to materialize both the high efficiency and the high speed response characteristics at the same time, to use a semi-insulating semiconductor layer doped with Fe which has electron-capturing characteristics as the buried layer is effective. FIG. 10A illustrates a semiconductor optical element (buried semiconductor laser) which uses this Fe-doped semiconductor layer as a buried layer.
In FIG. 10A, a p-type cladding InP layer 52, an active layer 53, and an n-type cladding InP layer 54 are laminated in the stated order from the bottom on an InP substrate 51 to form a mesa-stripe-shaped laminate. A buried layer is formed on each side of the laminate. In the buried layer, a first p-type InP layer 55, a first n-type InP layer 56, and an Fe-doped semiconductor layer 57 are laminated in the stated order from the bottom. An n-type contact layer 58 is formed on the laminate and the buried layers. The first p-type InP layer 55 and the Fe-doped semiconductor layer 57 are provided in contact with each other.
Here, if the Fe-doped semiconductor layer 57 is formed so as to be in contact with a semiconductor layer containing Zn which is a typical dopant of a p-type semiconductor, because Fe and Zn vigorously diffuse into each other, the layer changes into a p-type semiconductor layer. In this case, as illustrated in FIG. 10A, leakage current paths which is irrelevant to action of the semiconductor optical element are formed in the buried layers, and thus, there are problems that reactive current increases to decrease the efficiency and the high speed response characteristics are deteriorated.
In order to solve those problems, methods in which an Fe-doped semiconductor layer is grown so as not to be in contact with a p-type semiconductor layer, in which an Fe-doped semiconductor layer is isolated by an n-type semiconductor layer, and the like are proposed. FIGS. 10B and 10C illustrate semiconductor optical elements (buried semiconductor lasers) in which an Fe-doped semiconductor layer is grown so as not to be in contact with a p-type semiconductor layer. FIG. 10D illustrates a semiconductor optical element (buried semiconductor laser) in which an Fe-doped semiconductor layer is isolated by an n-type semiconductor layer.
In FIG. 10B, the buried layer is the same as the buried layer illustrated in FIG. 10A, but the first p-type InP layer 55 and the Fe-doped semiconductor layer 57 are not provided in contact with each other. Further, in FIG. 10C, the buried layer has, in addition to the buried layer illustrated in FIG. 10B, a second n-type InP layer 59 and a second p-type InP layer 60, and the first p-type InP layer 55 and the Fe-doped semiconductor layer 57 are not provided in contact with each other.
Further, in FIG. 10D, the buried layer is the same as the buried layer illustrated in FIG. 10C, but the Fe-doped semiconductor layer 57 is isolated by an n-type semiconductor.
When the Fe-doped semiconductor layer 57 is grown so as not to be in contact with a p-type semiconductor layer, as illustrated in FIGS. 10B and 10C, there is a problem that, because there are the plurality of n-type semiconductor layers, reactive current increases to decrease the efficiency.
When the Fe-doped semiconductor layer 57 is isolated by an n-type semiconductor layer, as illustrated in FIG. 10D, there is a problem that, because the semi-insulating Fe-doped semiconductor layer 57 is surrounded by the n-type semiconductor layers, the potential of an upper portion of the first p-type InP layer 55 is almost the same as the potential of a lower portion of the second p-type InP layer 60, a parasitic capacitance of the buried layer increases, and the high speed response characteristics are deteriorated.
More specifically, in a semiconductor optical element in which an Fe-doped semiconductor layer is used as a buried layer, there are problems that, due to interdiffusion between Fe and Zn and the like, reactive current increases to decrease the efficiency and the high speed response characteristics are deteriorated.
Therefore, in order to solve those problems, it is proposed that a semi-insulating semiconductor layer doped with Ru which causes almost no interdiffusion is used as a buried layer (see, for example, Japanese Patent No. 4249222).
By using the Ru-doped semiconductor layer, in forming the buried layer, it is not necessary to limit the kind of the semiconductor layer to be in contact therewith, and thus, design flexibility may be improved. Further, because an Ru-doped semiconductor layer has a characteristic to capture not only electrons but also holes, reactive current is also effectively suppressed.
However, because crystal growth conditions of an Ru-doped semiconductor layer for obtaining a satisfactory surface state as that of an Fe-doped semiconductor layer are very strict, an Ru-doped semiconductor layer lacks stability, and it is known that this tendency is conspicuous particularly when the Ru-doped semiconductor layer is thick. Further, because the crystal growth conditions are strict, it may be difficult to obtain a high activation rate and a high resistivity. Therefore, there is a problem that it may be difficult to, in order to decrease the parasitic capacitance and to materialize high speed response characteristics, grow a thick Ru-doped semiconductor layer and to exert a current blocking function only by the Ru-doped semiconductor layer.
Further, in order to solve problems in relation to a semiconductor optical element in which an Fe-doped semiconductor layer is used as a buried layer, to use a low carrier concentration semiconductor layer as a buried layer is proposed (see, for example, Japanese Patent Application Laid-open No. Hei 08-213691 and U.S. Pat. No. 5,636,237).
FIG. 11 illustrates a semiconductor optical element (buried semiconductor laser) in which such a low carrier concentration semiconductor layer is used as a buried layer.
In FIG. 11, the p-type cladding InP layer 52, the active layer 53, and the n-type cladding InP layer 54 are laminated in the stated order from the bottom on the InP substrate 51 to form the mesa-stripe-shaped laminate. The buried layer is formed on each side of the laminate. In the buried layer, the first p-type InP layer 55, the first n-type InP layer 56, the undoped i-type InP layer 61, and the second p-type InP layer 60 are laminated in the stated order from the bottom. The n-type contact layer 58 is formed on the laminate and the buried layers.
When such a low carrier concentration semiconductor layer (undoped i-type InP layer 61) is used as a buried layer, because existence of the undoped i-type InP layer 61 expands a depletion layer and the parasitic capacitance of the buried layers is decreased, the high speed response characteristics may be improved.
However, such conventional art has the following problems.
In the semiconductor optical element disclosed in Japanese Patent Application Laid-open No. Hei 08-213691 and U.S. Pat. No. 5,636,237, when the low carrier concentration semiconductor layer is grown in the buried layers on both sides of the mesa-stripe-shaped laminate, the low carrier concentration semiconductor layer is abnormally grown in proximity to the mesa-stripe-shaped portion (in particular, between the n-type semiconductor layer of the buried layers and the n-type cladding semiconductor layer or the n-type contact layer), and further, it is known that this tendency is conspicuous when the low carrier concentration semiconductor layer is thick.
Therefore, in this case, because the low carrier concentration semiconductor layer exhibits properties of a weak n-type semiconductor, as illustrated in FIG. 11, leakage current paths are formed in the buried layers, and thus, there is a problem that reactive current increases to decrease the efficiency. Further, when the low carrier concentration semiconductor layer is grown so as to be thin in order to suppress a reactive current flow through the buried layers, there is a problem that the parasitic capacitance of the buried layers increases to deteriorate the high speed response characteristics.
It is to be noted that the adverse effect of the increase in the reactive current is exerted not only on a single semiconductor optical element having only one function but also similarly on an integrated semiconductor optical element which quests for high speed response characteristics and power savings. Because the structure of an integrated semiconductor optical element is complicated, to suppress the reactive current therein is difficult in many cases. Further, because, in an integrated semiconductor optical element, semiconductor regions having different functions from one another and integrated so as to be adjacent to one another have structures different from one another, and the structures may not be greatly changed, to suppress leakage current to the integrated semiconductor regions is difficult in many cases.
Here, as an example, an integrated semiconductor optical element in which a waveguide semiconductor layer is formed by butt joint regrowth with respect to a semiconductor laser is illustrated in FIG. 12.
In FIG. 12, the active layer 53 and the n-type cladding InP layer 54 are laminated in the stated order from the bottom on the p-type cladding InP layer 52 which is laminated on an InP substrate to form a mesa-stripe-shaped laminate. An undoped type semiconductor layer 63 and an undoped i-type InP layer 61 are laminated in the stated order from the bottom on side portions of the laminate, using an insulating film mask 62 for selective growth.
In the integrated semiconductor optical element illustrated in FIG. 12, there is a current path from the upper cladding layer of the semiconductor laser portion to the upper cladding layer of the waveguide portion. In particular, when the upper cladding layer of the semiconductor laser portion is an n-type semiconductor, it is difficult to suppress the leakage current.